Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles.We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed.An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.     

A simple method to reduce discharge of sewage microorganisms from an Antarctic research station

The majority of coastal Antarctic stations release untreated sewage into the near-shore marine environment. This study examined bacterial reproduction within the temporary sewage-holding tanks of Rothera Research Station (Adelaide Island, Antarctic Peninsula) and monitored sewage pollution in the local marine environment. By continuously flushing the sewage-holding tanks with cold seawater we inhibited microbial reproduction and decreased the numbers of bacteria subsequently released into the sea by >90%. The widespread use of this simple method could significantly reduce the numbers of faecal coliform and other non-native microorganisms introduced into the Antarctic marine environment.     

Magnetohydrodynamics of stellar interiors

David Hughes, Robert Rosner and Nigel Weiss describe what was achieved during a programme on stellar magnetic fields at the Isaac Newton Institute in Cambridge. Over a four-month period more than 90 participants visited the Institute for a mixture of structured workshops and informal collaboration.     

Bentonites in the Chalk of central eastern England and their relation to the opening of the Northeast Atlantic

Within the post-Cenomanian Chalk sequence of central eastern England about 20 thin persistent marl horizons occur. These marls contain from 30 to 65% non-carbonate material the bulk of which is composed of a well crystallised Mg-rich smectite with a notable paucity of illite and quartz. The Chalk matrix above and below the marls contains up to 2.0% of non-carbonate matter which again is dominated by the presence of an Mg-rich smectite but which contains a larger proportion of illite and quartz than occurs in the marls. For both sets of non-carbonate materials the excellent crystallinity and Mg-rich nature of the smectite strongly suggest that it was formed by the in situ alteration of an aeolian transported volcanic ash, i.e. it is a true bentonite. Quartz and illite in the Chalk matrix are probably detrital components.Relative to a standard shale, non-carbonate materials from the Chalk marl horizons exhibit a highly anomalous trace element pattern with accentuated levels of Zr, Nb and Th but depleted levels of Ba, Rb and K. This trace element chemistry corroborates a volcanogenic origin for the smectite insofar as it is not explicable in terms of any realistic sedimentary process. The chemistry is best interpreted in terms of the magnetic processes responsible for the formation of the original ash precursor of the marls. It appears that this precursor had a pantelleritic composition and therefore originated from a source overlying an area where incipient continental rifting was taking place.Micromorphological examinations prove that residues from the Chalk marls contain small amounts of pyroclastic debris, including pumice and fragments of sideromelane glass. The morphology of the fragments suggests that the volcanic eruptions responsible for the ejection of the original ash into the air were of the shallow-water phreatomagmatic (Surtseyan) type.The distribution of marls in the Chalk of eastern England suggests that the ashfalls responsible for their origin were of the tropospheric type and were drawn by westerly winds from a centre to the west or northwest of mainland Britain. Of the numerous volcanic centres that lie in offshore areas to the west, the Anton Dohrn Seamount in the Rockall Trough appears to a likely centre of origin for the ashfalls. However, ashfalls responsible for the formation of the traces of smectite in the Chalk matrix were probably of the general background (stratospheric) type and may have been drawn from more distant centres.Volcanism at the Anton Dohrn Seamount was related to tensional effects in the continental crust that immediately preceded the formation of the Rockall Trough and the separation of Greenland from northwestern Europe during Tertiary times. Activity at the centre, and Cretaceous bentonites derived therefrom, therefore represent the earliest stages in the development of the Thulean igneous province.     

The characteristics of Pc 3 and Pc 4 geomagnetic micropulsations recorded at Sanae,Antarctica

Digital dynamic spectra of micropulsations recorded at SANAE (L ～ 4) show that Pc 3 pulsations have frequencies which decrease throughout the day. Both the onset frequency and the rate of decrease of frequency depend on the level of magnetic activity during the previous night. The variation of Pc 3 amplitudes and frequencies is explained in terms of the position of the plasmapause and the associated Pc 3 resonance region in the plasmatrough.For Pc 4 pulsations a constant frequency is observed on most days and it is not possible to infer the presence of a Pc 4 resonance region.     

Internal stratigraphic relationships in the Etendeka group in the Huab Basin, NW Namibia: understanding the onset of flood volcanism

The Etendeka Igneous Province in NW Namibia forms the eastern most extent of the Paraná–Etendeka Flood Basalt Province and, despite only covering about 5% of the Paraná–Etendeka, has been the focus of much interest, due to its extremely well exposed nature. The Huab Basin in NW Namibia forms the focus of this study, and formed a connected basin with the Paraná throughout Karoo times (late Palaeozoic) into the Lower Cretaceous. It contains a condensed section of the Karoo deposits, which indicate early periods of extension, and Lower Cretaceous aeolian and volcanic Etendeka deposits, which have their correlatives in the Paraná. In the Huab Basin, the volcanic rocks of the Etendeka Group consists of the Awahab and Tafelberg Formations, which are separated by a disconformity. Detailed examination of the Awahab Formation reveals an additional disconformity, which separates olivine-phyric basalts (Tafelkop-type) from basalt/basaltic andesites (Tafelberg-type) marking out a shield volcanic feature which is concentrated in an area to the SE of the Huab River near to the Doros igneous centre. Early volcanism consisted of pahoehoe style flows of limited lateral extent, which spilled out onto aeolian sands of an active aeolian sand sea 133 million years ago. This sand sea is equivalent to the sands making up the Botucatu Formation in the Paraná basin. The early expression of flood volcanism was that of laterally discontinuous, limited volume, pahoehoe flows of Tafelkop-type geochemistry, which interleaved with the aeolian sands forming the Tafelkop–Interdune Member basalts. These basalts are on-lapped by more voluminous, laterally extensive, basalt/basaltic andesite flows indicating a step-up in the volume and rate of flood volcanism, leading to the preservation of the shield volcanic feature. These geochemically distinct basalts/basaltic andesites form the Tsuhasis Member, which are interbeded with the Goboboseb and Sprinkbok quartz latite flows higher in the section. The Tsuhasis Member basalts, which form the upper parts of the Awahab Formation, are of Tafelberg-type geochemistry, but are stratigraphically distinct from the Tafelberg lavas, which are found in the Tafelberg Formation above. Thus, the internal stratigraphy of the flood basalt province contains palaeo-volcanic features, such as shield volcanoes, and other disconformities and is not that of a simple layer-cake model. This complex internal architecture indicates that flood volcanism started sporadically, with low volume pahoehoe flows of limited lateral extent, before establishing the more common large volume flows typical of the main lava pile.     

Solar Eclipses and the Surface Properties of Water

Earth, Moon, and Planets - During four solar eclipse events (two annular, one total and one partial) a correlation was observed between a change in water surface tension and the magnitude of the...